Actinium and radium fluxes from the seabed in the northeast Pacific Basin

Five sediment cores were collected along a cruise tract from Hawaii to Alaska in August 2017 (C-Disk-IV cruise) with the objective of characterizing the behavior of Ac-227, Ra-228, and Ra-226 and their fluxes into the overlying water column, information that is essential to the interpretation of the distribution of these tracers in the ocean, for example, as measured on GEOTRACES cruises. Solid phase profiles of these isotopes were measured, and reaction-transport models were applied that incorporated molecular diffusion, bioturbation, sedimentation, distribution coefficients (kd), and the fraction of each isotope released to pore water by parent decay (called F). Fits to these profiles used kd values determined in lab experiments for C-Disk-IV sediments. Ra kd values (1000-3000 mL g(-1)) agreed with previous estimates for deep-sea sediments, and Ac kd values (3500-22,000 mL g(-1)) correlated with those for Ra but were about 7 times greater. Two independent approaches were used to quantify the benthic fluxes of Ac-227 and 228Ra in the Northeast Pacific: (1) use of solid phase profiles with a reaction-transport model, as well as integrated downcore daughter parent deficiency; and (2) direct measurement of fluxes based on core incubation. The two independent methods agreed within uncertainty, and the average Ac-227 and Ra-228 sediment fluxes for the Northeast Pacific are 90 & PLUSMN; 20 and 600 &200 dpm m-2-yr(-1), respectively. The 226Ra sediment flux was only determined by the former approach, and the flux calculated in this study is similar to previous work in the North Pacific, averaging 1300 & PLUSMN; 200 dpm m-2-yr(-1). This is over 2x higher than the water column inventory of Ra-226 in this region (600 dpm m-2-yr(-1)), and indicates the importance of lateral 226Ra export from the N. Pacific. The largest 227Ac and Ra isotope fluxes in the study area are near the center of the Northeast Pacific (37 degrees N). Smaller Ac-227, Ra-228 and Ra-226 fluxes occur north of 40 degrees N, primarily due to dilution of their Pa and Th ancestors by higher sediment accumulation rates.

Automated summary

In August 2017, five sediment cores were collected along a cruise tract from Hawaii to Alaska to study the behavior and fluxes of Ac-227, Ra-228, and Ra-226. The study found that the sediment fluxes of Ac-227 and Ra-228 are relatively large in the Northeast Pacific. However, the sediment flux of Ra-226 is relatively small in this region, indicating the significant lateral export of Ra-226 from the North Pacific.